Polyurethane prepolymers may be used to provide hot melt adhesives that cure upon exposure to moisture. Such adhesives are solid at room temperature but melt to a viscous liquid when heated to moderate temperatures (for example, less than 135.degree. C.). The adhesive is applied in a molten state to a substrate, and the substrate is mated to a second surface. As the adhesive cools, it rapidly "sets up" (i.e., solidifies and/or crystallizes), thereby forming a bond between the substrate and the second surface. "Set time" defines the time required for the moisture-curable, hot melt adhesive to crystallize or solidify and form a bond between the substrate and second surface once they have been mated.
Another related property which can be used to describe a moisture-curable, hot melt adhesive is "creep resistance". "Creep resistance" refers to the capacity of a bond to sustain a constant load applied under a constant stress. A bond which creeps moves slowly under stress. Items bonded together by an adhesive which tends to creep will move, may not stay in place, and may even fail. A bond which is creep resistant combats this tendency. Preferably, the bond is creep resistant within minutes of bond formation. In this case, the bond may be described as providing "initial" creep resistance.
"Open time" is another property which can be used to characterize a moisture-curable, hot melt adhesive and refers to the amount of time available to form bonds once the adhesive is applied to a substrate. Once the open time has been exceeded, the substrate and a second surface typically cannot be successfully mated.
Typical uses for moisture-curable, hot-melt adhesives include the construction of signs, advertising displays, and various laminates such as table tops, sound absorption panels, countertops, wall partitions, and the like. For these applications, it is desirable to use a moisture-curable, hot melt adhesive that provides a short set time, initial creep resistance, and a long open time. An adhesive that provides a short set time and initial creep resistance obviates the need to clamp the bonded materials together and/or store the parts for an extended period of time to allow the adhesive to build strength subsequent to further processing. Either of these requires additional processing which causes delays in building the finished product. In addition, initial creep resistance enables the bonded items to stay in place despite the application of a constant stress shortly after the parts are mated. This is especially important, for example, in applications where parts are attached to substrates such as door panels which are in a vertical position. In this case, unless the adhesive provides initial creep resistance, the force of gravity may cause the part to substantially shift downward. Other examples of applications where initial creep resistance is important include those where sheets or materials are bonded to curved surfaces or to articles which exert "memory forces". "Memory forces" may be exerted, for example, by a roll of material that has been tightly wound and subsequently unwound, the material having a tendency to curl due to its "memory" of being in a roll. Such forces can cause the two mated materials to shift, unless the adhesive provides initial creep resistance. A long open time allows the moisture-curable, hot melt adhesive to be coated onto a large surface and subsequently attached to a second surface before the adhesive loses its ability to form bonds. In addition, a long open time also allows the moisture-curable, hot melt adhesive to be spray coated. In general, spray coating exposes a larger surface area of adhesive to cooling; and if the adhesive does not possess a sufficiently long open time, it may prematurely lose its ability to form bonds.
Numerous references describe ways to improve the properties of moisture-curable, hot melt adhesives comprising polyurethane prepolymers. For example, U.S. Pat. No. 5,021,507 (Stanley et al.) describes the addition of urethane prepolymers to low molecular weight polymers formed from ethylenically unsaturated monomers which do not contain active hydrogen. The reference states that such a composition provides hot melt adhesives and gasketing compositions which have improved initial cohesive strength as well as improved strength after aging.
U.S. Pat. No. 5,173,538 (Gilch et al.) describes a hot melt polyurethane composition comprising a mixture of at least two polyurethane prepolymers, the first polyurethane prepolymer having a glass transition temperature above room temperature and a second polymer or polyurethane prepolymer having a glass transition temperature below room temperature. The reference states that the composition provides a fast-setting adhesive that cures with atmospheric moisture to give flexible bonding over a wide temperature range.
U.S. Pat. No. 5,115,073 (Meckel et al.) describes a polyurethane system containing isocyanate groups prepared by reacting at least one polyester diol having a melting point of 30 to 60.degree. C. and a molecular weight of 800 to 8,000; a polyester diol having a melting point of 65 to 150.degree. C. and a molecular weight between 600 and 8,000; a diol having a molecular weight less than 600; and a polyisocyanate. The reference states that the crystallization rate of such systems is greatly accelerated.
Other references which describe ways to improve the properties of moisture-curable, hot melt adhesives comprising polyurethane prepolymers include, for example, JP 3-88886; U.S. Pat. No. 4,889,915; U.S. Pat. No. 5,036,143; U.S. Pat. No. 5,266,606; U.S. Pat. No. 5,034,453; U.S. Pat. No. 4,985,535; U.S. Pat. No. 5,019,638; U.S. Pat. No. 5,155,180; WO 9207015; JP 6-158017; and JP 6-158014. While many of these references describe ways to improve at least one of the properties of initial creep resistance, set time, and open time, or related properties thereof, a need exists for a moisture-curable, hot melt adhesive that provides an acceptable balance of all three properties.